1. Field of the Invention
The invention relates to a rubber cylinder sleeve of the type having a carrier sleeve which can be expanded using air and a rubber covering on the carrier sleeve.
2. Description of the Related Art
Rubber cylinder sleeves are known in which a carrier sleeve is provided with a rubber covering, the rubber covering comprising three or more layers. EP 0 421 145 B1 is to be cited here by way of example.
In rotary offset presses, as is known, the printing image is transferred from the plate or forme cylinder onto the rubber cylinder and from the latter onto the paper running over the impression cylinder. It is only possible to transfer the ink, both from the printing forme onto the rubber sleeve and from the rubber sleeve onto the paper, if a certain minimum pressure is present, what is known as the line pressure between the blanket cylinder and plate or forme cylinder or impression cylinder.
Here, a problem arises for quality assurance from the demand for ever higher productivity, and as a result of the efforts to produce impression cylinders which are as light and cost-effective as possible. Especially what is referred to as channel-less printing, in particular therefore the sleeve technique, which is distinguished by a printing forme applied without a channel onto a sleeve and/or a rubber blanket applied without a channel, allows the rigidity to be reduced because of the lessened oscillation excitation as a result of the missing cylinder channels. As a result, the length-to-thickness ratio of the printing cylinders, or their relative rigidity with regard to deflection, becomes ever more unfavorable. The consequence of this is that, during printing operation, the shape and position of the printing cylinders with respect to one another change in an undesired manner, i.e. the printing cylinders are deflected.
The positional change as a consequence of a deflection changes the printing pressure, i.e. the setting pressure between the printing cylinders interacting in the printing unit, this setting pressure becoming non-uniform as seen across the cylinder width. This printing pressure is usually determined in numerical values by measuring what is referred to as the imprint width, i.e. the width of the zone which defines the contact area of the cylinders when the cylinders are thrown onto one another, i.e. moved to the pressure position. This measurement is particularly simple in offset printing, since here one cylinder of a pair of cylinders always has a compressible (soft) surface.
As a result of the mechanical misalignment remaining with this, it is known that folds can form in the conveyed paper web in the press nip of the rubber cylinder if the paper web is moving here with an irregular speed profile across the width because of the above-described positional change, the center of the paper web moving more quickly than the outside of the web, which leads to the formation of folds. The web transport behavior in web-fed offset presses is critically influenced, however, by the conveying characteristics of the rubber blankets. In sleeve presses, in particular, it is the case that folds may form across the web width, which impair the page register. To remove this problem, for example in DE 44 36 973 A1, rubber sleeves have already been configured with a concave or convex surface geometry across the web width, i.e. a thickness profile varied by the circumferential surface assuming a convex or concave shape on the blanket cylinder in the axial direction of the cylinder.
In this document, it is also proposed to vary the compressibility or the rigidity in the axial direction, radial variables being concerned in each case here, i.e. the radial compressibility and the radial rigidity are taken as a basis (as has been the case in the prior art for a long time), but these variables should be different in the axial direction, according to this document, as has been mentioned respectively in the radial direction, i.e. as seen transversely through the plane of the rubber blanket.
Although the deflection between a blanket cylinder and a plate or forme cylinder can be compensated for by a convex profile of the blanket-cylinder surface, the contact is impaired, on the other hand, between the two blanket cylinders in a printing unit for recto and verso printing. This has a negative influence on both the web transport and the ink transfer to the paper web. Although the contact between the two blanket cylinders in the press nip is improved by a concave blanket-cylinder surface, the ink transfer from the plate or forme cylinder to the blanket cylinder is then impaired. The results are similar if the compressibility or rigidity is changed in the axial direction (seen radially).
Moreover, the above-described measures to influence the conveying characteristics have the disadvantage that the radial compressibility or radial rigidity is modified over the sleeve length, which in turn impairs the print quality, in particular the tonal value gain.